1. Field of the Invention
The present invention relates to medical devices, systems and methods for bone fixation. Specifically, embodiments of the invention are related to stabilizing adjoining vertebrae in the cervical, thoracic, and lumbosacral spine. In addition, embodiments of the invention are related to fusion or stabilization of vertebrae in the lumbar spine to alleviate axial back pain. Embodiments of the invention are also related to improving minimally invasive surgical (MIS) approaches to pedicle screw fusion by reducing the number and size of incisions and the size of the medical instruments inserted therein.
2. Description of the Related Art
While some lower back conditions can be ameliorated with non-surgical approaches, spinal fusion is recommended for certain conditions when non-surgical approaches fail. Non-surgical approaches include medications, physical therapy, chiropractic treatment, traction, epidural steroid injections, facet blocks or rhizotomy, weight loss, smoking cession, and acupuncture. Conditions that commonly serve as indications for spinal fusion or stabilization surgery can be divided generally into three categories: (i) trauma induced, (ii) curvature, and (iii) degenerative.
Trauma induced conditions include fractures and ligamentous injuries. Fractures typically result from an unfortunate incident involving an extraneous force or fall but may also arise from pathologic conditions, such as cancer or osteoporosis. Fractures are often compressive in nature and typically lead to a pathological curving of the spine resulting in a loss of the natural lordotic curvature in the lumbar and cervical spine, known as kyphosis. Fractures of the spine also occur with translational or rotational forces perpendicular to the axis of the spine. These forces result in fractures of the facet or pars interarticularis (pars). If the external forces are large enough, vertebrae can collapse resulting in a burst fracture that can injure all 3 columns of the vertebrae (anterior, middle, and posterior columns). Many traumatic injuries can heal without surgery, but unstable injuries that pose a risk for neurologic injury and/or pain require stabilization through a procedure such as fusion.
A condition called spondylolisthesis characterized by slippage of the spine bones or vertebrae relative to one another can result from fractures of the pars interarticularis (pars fracture) known as spondylolysis. Spondylolisthesis can also develop from malformation of the facet joints by degenerative arthritis as well as congenital malformation and pathologic conditions such as tumors. If the pars on both sides are fractured, then the spinous process and lamina are essentially completely disconnected from the pedicle and vertebral body. This large fragment is called the Gill body. Pars fractures are actually common in people of all ages (often acquired in the teenage years). While, many of these patients are mildly symptomatic and do not require surgery, those with progressive symptoms may require surgical decompression with or without fusion. Spondylolisthesis results in misalignment of the spine and increases the risk of a nerve becoming entrapped. Nerves travel within the spinal canal bounded by the vertebrae and their roots protrude from the curved openings in the sides of the vertebrae called foramina (singular is foramen). These spinal nerves are suspected to be the source of back and radicular pain when they become entrapped or when the nerve endings become irritated by irregular or abrasive motion around a disc, bone, or joint. Spondylolisthesis can also aggravate or be accompanied by degeneration of disc or facet joint which can lead to axial back pain.
The normal curvature of the lumbar and cervical spine is lordosis, where the posterior aspect of these spinal levels forms a concave curve. The thoracic spine normally has a kyphotic or convex curve. Curvature conditions include straightening of the natural curvature as well as abnormal lordosis, abnormal kyphosis or lateral/rotational bending called scoliosis. Curvature conditions can occur idiopathically during adolescence, e.g., adolescent idiopathic scoliosis, or develop as a secondary problem in situations where spinal muscle activation is abnormal such as cerebral palsy, spina bifida, or tethered cord syndrome. Abnormal spinal curvature is common in spinal degeneration when the discs and joints degenerate asymmetrically leading to a progressive curvature (scoliosis, kyphosis, or lordosis) as the biomechanics of the spine are disrupted. Curvature conditions also occur after trauma with compression or burst fractures or with ligamentous injury. Additionally, curvature conditions can occur iatrogenically after previous spinal surgery where the anatomy and biomechanics of the spine have been altered. Such situations include the removal of the posterior tension band after laminectomy as well as the alteration of physiologic movement after spinal fusion leading to adjacent level compensation and degeneration. Curvature conditions lead to abnormal biomechanical stress on the discs and facet joints accompanied by compensatory measures such as facet or ligamentous hypertrophy. Patients can develop both axial back pain and radicular pain. In patients who have failed conservative therapy and bracing, surgery can be effective. Surgery in these conditions includes decompression of nerve or spinal cord compression as well as fusion or stabilization. Curvature can be corrected through surgery, and fusion prevents further curvature from developing.
Degenerative conditions include spinal arthritis and recurrent disc herniation. Spinal arthritis is the most common indication for fusion and may exist in the form of severe disc degeneration (also called Degenerative Disc Disease, DDD) or facet disease. Degenerative arthritis can also be a cause of spondylolisthesis in addition to traumatic fractures discussed above. Degenerative conditions are generally accompanied by nerve compression causing radicular pain in the distribution of the nerve's receptive field, which usually correlates with and is manifested in arm or leg pain. Pure nerve compression syndromes such as herniated nucleus pulposus (herniated discs) or foraminal stenosis (narrowing of the side foramina canals through which the nerves pass) can often be treated with decompression without fusion. Pure disc degeneration syndromes can be treated with fusion without decompression of the nerves. However, most commonly disc degeneration occurs in combination with nerve compression causing both axial back pain and radicular limb pain. In these circumstances fusion surgery is combined with nerve decompression surgery.
Fusion functions to eliminate motion in the disc space and facet joints between adjacent vertebrae. The vertebrae provide the rigid structural framework of the spine and the fibrocartilaginous disc space acts as a cushion or shock-absorber. Degradation of the disc space can distort alignment and alter the biomechanical cushion that the disc affords the adjacent vertebrae. This degradation alters the forces impacted upon the vertebrae and results in axial back pain. Fusion is designed to eliminate movement between adjacent vertebrae by either forming a solid bridge of bone across the disk space and/or creating new bone formation in the posterolateral space to provide stabilization, rigidity, and strength. Sometimes fusion involves a bone graft taken from another location in the body (e.g., autograft from the iliac crest in the pelvis) or from an external source, e.g., allograft. Physicians commonly refer to the level of a fusion. A single level fusion involves stabilizing the two vertebral bones adjacent to a diseased disc. A two-level fusion involves stabilizing three adjacent vertebral bones spanning two problematic disc spaces. Each vertebra makes contacts (joints) with adjacent vertebrae at three points, the paired facet joints located posteriorly and the intervertebral disc located anteriorly. Thus, lumbar fusion can be directed either at the posterior facet joints or at the anterior interbody/disc space or both. When an anterior interbody fusion is performed in combination with posterior fusion, the procedure is termed 360° fusion. One commonly used technique of posterolateral fusion is pedicle screw fusion where screws are directed into the pedicle portions and the bodies of adjacent vertebrae and then rods are connected to the screws across the disc spaces. The screws and rods hold the adjacent vertebrae motionless relative to one another and allow the bone graft that is placed either in the interbody (disc) space or in the posterolateral space to grow into solid bone. Conventional pedicle screws and rods are metal, typically titanium (Ti) alloy but have been made from stainless steel as well. Recently rods have been made from a minimally flexible polymer called polyetheretherketone (PEEK).
Interbody fusion involves placing one or more spacers (typically pre-loaded with bone graft material) within the interbody (disc) space between bony vertebral bodies after the degenerated disc has been cleaned out and removed. Spacers are made from bone grafts, titanium, carbon fiber, or polymers such as PEEK. Interbody fusion can be performed through several approaches including: an anterior approach (anterior lumbar interbody fusion, ALIF), a posterior approach (posterior lumber interbody fusion, PLIF, or transforaminal lumbar interbody fusion, TLIF), or a lateral approach (direct lateral interbody fusion, DLIF™—Medtronic, or extreme lateral interbody fusion, XLIF™—Nuvasive). The aim of these approaches is to remove the degenerated disc and replace the disc with material that induces bony fusion. Alternatively the disc can be replaced with an artificial joint/disc (discussed below). Each of these interbody approaches has advantages and disadvantages. Anterior procedures require a retroperitoneal dissection and risk injury to the large blood vessels anterior to the lumbar vertebrae. Also injury to the nerve plexus anterior to the vertebrae can result in sexual dysfunction. The lateral approach is promising but is limited to the upper and mid lumbar levels (rostral to L5,S1) because of obstruction by the iliac crest. The posterior interbody approach is more time consuming and typically requires more muscle dissection and retraction. However, the posterior approach allows the placement of the interbody graft, posterior pedicle screw fusion, and decompression of nerves all to occur through the posterior incision(s).
Although anterior and lateral approaches can be performed stand-alone (without posterior instrumentation), many surgeons will back-up or supplement anterior or lateral interbody fusions by placing pedicle screws posteriorly after the interbody cage or graft has been placed. This 360° fusion limits movement more than just an isolated anterior or posterior fusion, and fusion rates are increased. However in ALIF and lateral interbody (DLIF, XLIF) cases, two sets of incisions are required for a 360° fusion.
The posterior approaches (TLIF and PLIF) allow an interbody fusion, pedicle screw fusion, and neural decompression to be done all through the same posterior incision(s). In the TLIF, a single large interbody spacer is inserted on the side ipsilateral to the patient's symptomatic side after neural decompression is completed. If both sides are symptomatic then decompression is required on both sides. A PLIF is performed by placing two interbody spacers, one on each side. Posterior procedures may be done according to: (i) an invasive open procedure in which a large incision and/or several incisions are made, (ii) a percutaneous approach in which small incisions and/or few incisions are made, and potentially (iii) an endoscopic approach in which small incisions are made and all tools and devices are inserted through portals with visualization provided on an external monitor.
As an alternative to fusion, recent advances in interbody stabilization have resulted in the development of artificial disc technology. Artificial discs replace the degenerated discs and allow continued motion at the joint. Both cervical and lumbar artificial discs have been developed. Additionally, dynamic stabilization techniques have been developed for the posterior spine. These posterior techniques utilize pedicle screws and a dynamic rod. Typically the dynamic rod has a mechanism to bend under certain loads or forces, thereby absorbing some stress and strain that is applied to the spine. The advantage of dynamic stabilization is that motion is preserved in the spine. However, the durability of these systems may be an issue. In fusions, the bone graft (interbody or posterolateral) eventually fuses the vertebrae eliminating the need for the spinal instrumentation (screws and rods). However in dynamic stabilization, fusion does not occur, so the screws and dynamic rods will always be subjected to the strain and forces of the spine. Over time the possibility of loosening of the pedicle screws or mechanical failure may increase. Sometimes the use of a slightly flexible rod such as a rod made of PEEK may actually increase fusion by reducing stress shielding. Stress shielding occurs when rigid fusion constructs shield the vertebral bone in contact with the bone graft from the stresses required to form and remodel bone.
Posterior lumber stabilization (fusion and dynamic stabilization) techniques have evolved into minimally invasive approaches because such minimized exposures reduce patient morbidity and facilitate patients' recovery to function. Blood loss and hospital stays are shorter. The process of performing a minimally invasive pedicle screw fusion is the same as that for dynamic stabilization and involves two basic parts. First, screws are placed percutaneously through the pedicle into the vertebral body. For minimally invasive systems, cannulated screws are placed percutaneously over a fluoroscopically (an X-ray that can be seen on a video screen) guided guidance element. Generally, two screws are used on each vertebral body being fused, one on a right side and the other on a left side. The second part of the process involves connecting the screws with a rod and locking the rod and screws together. In dynamic stabilization, the rod or rod-like device (flexible connector) is bendable, but the process of inserting this bendable rod is the same as that for fusion. For example, a rod-like device (flexible connector), like a rod, fits within the screw heads, but may also include an element (a shock absorber, a spring, etc.) that allows some motion. The variations between different minimally invasive systems mostly arise in the method of placing the rod and locking the rod with the screws through a minimal incision.
Before the intervertebral body spacer is inserted, the damaged or degenerated disc within the disc space must be removed. In the TLIF approach, the disc space is accessed through a facetectomy in which the foramen around the nerve roots is opened with a bone-cutting tool such as an osteotome or a high speed drill. In the PLIF approach, laminectomies or laminotomies are performed to access the disc space. Both TLIF and PLIF allow for decompression of the spinal thecal sac and the nerve roots; however, the facetectomy in a TLIF allows the maximum decompression of the exiting nerve root on that side. With gentle retraction of the thecal sac, the disc space is easily accessed. Then the instruments used for clearing out the degenerated disc may be inserted into the disc space to complete the discectomy.
Following removal of the disc, the surgeon should prepare the bony surfaces, known as the end plates, of the vertebral bodies on each side of the disc that was removed. Peeling off the end plate with a tool such as a curette induces bleeding which stimulates healing and assimilation of the bone graft to be inserted into the interbody space. The spacer or cage that is to be inserted is typically constructed of bone, titanium, carbon fiber, or polymers such as PEEK. The spacer is usually hollow or at least porous to accommodate bone graft material therein. Bone inducing protein such as bone morphogenetic protein (BMP) is also commonly placed within the spacer. After placing the spacer and bone graft, the rods may be inserted into the pedicle screws and the screws can be tightened to lock the rods in place.
Typically the placement of the percutaneous screws is fairly straight forward. The insertion of the rod through the screw heads and locking of the rod with the screws are the steps that are currently most difficult through a minimal incision. In most of the minimally invasive surgery (MIS) systems used today, a guidance element, such as a wire, is placed percutaneously under fluoroscopic guidance through the pedicle. Percutaneous cannulated drills and screw taps are inserted over the guidance element/wire to prepare the tract through the pedicle and vertebral body for pedicle screw insertion. Dilating tubes and a guidance tube or a retractor system are often used to dilate and hold open the path around the guidance element through skin and muscle to reduce injury to muscle and tissue when pedicle screws and insertion tools are inserted. Pedicle screws are inserted over the guidance elements either with or without passage through a guidance tube/retractor. In order to place the rod and locking assembly into the screw heads, each screw head is associated with a tower that extends through the skin incision. The tower has to accommodate the rod and locking assemblies so it is typically larger than the maximum diameter of the screw head. Once the towers are in place, the rod is then inserted through one of a variety of methods. The leading MIS system is Sextant™ by Medtronic. In this system, the rod is placed by forming a pendulum like mechanism. The two or three towers (for one or two-level fusion, respectively) are coupled together to align the towers, and the rod is swung around through a separate incision superior or inferior to the towers in a pendulum fashion. Once the rod is swung in place, locking caps are placed through the towers and tightened. Alternatively, most of the other systems insert the rod through one of the towers and then turn the rod approximately 90° to capture the other screws in the other towers. Inserting the rod through the screw heads in a minimally invasive system is done blindly, e.g., without direct visualization of the screw head. Thus this process is sometimes tedious and frustrating.
The Sextant™ system and other systems that use towers are hindered by both the number of incisions required and the size of each incision. The use of a separate tower for each screw requires a separate incision for each tower, or a single incision long enough to accommodate two towers. The Sextant™ system also requires an additional incision for the rod, equaling six incisions (three on each side) for a single level fusion and eight incisions for a two level fusion. The other tower systems that use the direct rod insert and turn mechanism still require one incision for each screw and each incision has to be larger than the size of a tower through which the screws are inserted. Typically, each incision is at least 15 mm in length. When the sum of the lengths of all incisions on both sides are totaled, the total length of the current leading minimally invasive systems often are longer than the single midline incision of a traditional “open” approach for a single or two level pedicle screw fusion.
U.S. Pat. No. 7,306,603 entitled “Device and method for percutaneous placement of lumbar pedicle screws and connecting rods” by Frank H. Boehm, Jr., et al. and assigned to Innovative Spinal Technologies (Mansfield, Mass.), the entirety of which is hereby incorporated by reference, discloses a system of connecting a rod to the pedicle screws using a pin and recesses within the screw heads. According to this system the rod can pivot about a longitudinal axis of the pin between a first position in which the rod is parallel to the longitudinal axis of the screw (e.g., vertically oriented) and a second position in which the rod is transverse to that axis in order to bridge screws on adjacent vertebrae. The '603 patent teaches various guide systems (see FIGS. 5 and 6), rod holder systems (see FIGS. 8, 9, 10, and 11), and a rod guide system (see FIG. 12) but does not include a sleek, detachable system among them. Rather, the systems illustrated are tower-like with rather bulky dilators (80 and 86 in FIGS. 6 and 8), sheaths (81 in FIG. 6), and/or outer housing (120 in FIGS. 11 and 12).
U.S. Publication No. 2008/0140075 entitled “Press-On Pedicle Screw Assembly” by Michael D. Ensign and assigned to Alpinespine, LLC (American Fork, Utah), the entirety of which is hereby incorporated by reference, discloses attaching the rod to screw heads indirectly via a tulip assembly. The tulip assembly has a housing with an inner diameter smaller than an inner diameter of the screw head such that it is easily pressed into position upon the screw head. The rod is then placed by attaching directly to the tulip assembly after connecting the assembly to the screw head. The publication mentions using a Kirschner guidance element (or K-guidance element) for inserting both the pedicle screws and the tulip member (see [0030], [0032], and [0045]) but does not disclose how the rods are guided into position.
U.S. Publication No. 2008/0097457 entitled “Pedicle screw systems and methods of assembling/installing the same” by David R. Warnick and unassigned, the entirety of which is hereby incorporated by reference, like the '075 Publication, also discloses using a tulip assembly as an intervening means to join a rod to the screws. In this system, rather than a press-on locking mechanism, the structure is tightened by rotating an inner member and outer housing of the tulip assembly relative to one another.
U.S. Pat. No. 7,179,261 entitled “Percutaneous access devices and bone anchor assemblies” by Christopher W. Sicvol, et al. and assigned to Depuy Spine, Inc., the entirety of which is hereby incorporated by reference, describes one of the several tower systems for placement of pedicle screws percutaneously. The patent describes a situation where the angle of the screws intersect, and the towers may interfere with each other. This situation is rather typical in the lordotic lumbar spine, especially the lumbo-sacral (L5, S1) junction. In order to solve this problem, they describe cut-outs in the tubes so that two tubes can intersect. Given that the angles of the vertebrae are variable from patient to patient and the depth of the vertebrae from the skin is also highly variable, the variations on the cutouts would have to be numerous. Additionally, when two tubes intersect at the cutout as shown in FIG. 22B in the '261 patent, the edges of the cutout of one tube interferes or blocks off the lumen of the other tube, and vice versa. This occurs because the muscle and tissue surrounding the tubes will push the tubes together at the section of the cutouts thereby significantly reducing the lumen through which the rod and other elements are inserted. The only way to avoid this interference or blockage of the lumens is to keep the tubes separate which would necessitate a larger incision and would eliminate the need for cutouts in the first place.